The present invention relates to cardiac pacers; and more particularly, it relates to cardiac pacers which are adapted for implantation in a patient.
The present invention is seen as providing substantial improvements in many areas of conventional cardiac pacers, both in the functioning of implanted pacers and in the circuitry and hardware used to implement a design.
One way to improve current cardiac pacers, as expressed in the literature and known to persons skilled in the art, is to establish a normal cardiac rhythm between the contraction of the upper, smaller heart chamber or atrium, and the lower or main heart chamber, called the ventricle. In a normal heart, the atrium expands and contracts, forcing blood through the tricuspid valve into the associated ventricle, thereby helping to fill the ventricle so that when the ventricle contracts (normally about 150 milliseconds after contraction of the atrium), it is filled to normal capacity, and this augments the efficiency of the heart in the sense that more blood is pumped for each cardiac cycle. This also causes the ventricular rate to follow the atrial rate as established by the sino-atrial node, thus adjusting the pacing rate to the body demands.
The most common implantable cardiac pacer in commercial use today is a demand pacer--that is, it has a single electrode which is lodged in the apex of the ventricle and senses contraction of the ventricle (an R wave). A demand pacer then establishes a predetermined time out period or escape interval during which it tries to sense a natural R wave. If no natural R wave is sensed during that period (which may be approximately 833 milliseconds), a stimulating pulse is generated and transmitted to the ventricle. If a natural heartbeat is sensed, the time out period is reset to establish a new escape interval. Thus, in a cardiac pacer of this type, no attempt is made to synchronize the contraction of the ventricle with that of the atrium.
Early attempts were made to synchronize ventricular contraction with atrial contraction (called a P wave), particularly when the prevailing practice was to use open chest surgery to implant the cardiac electrodes. The electrodes were sutured to the walls of the atrium and the ventricle respectively. Current practice, however, has tried to avoid the use of open chest surgery with its accompanying trauma and risk, and therefore the prevailing practice is to introduce a single electrode which, as indicated, is lodged in the apex of the ventricle, and to employ a second or neutral electrode on the pulse generator casing for the circuitry which is normally lodged in the abdomen or chest pocket of the patient.
The magnitude of the electrical signal accompanying atrial contraction, that is the P wave, is quite small and easily masked by ambient electrical noise or artifacts, such as those generated by muscular activity. When an electrode is sewn directly to the wall of the atrium, a P wave can be detected. However, when it became generally accepted that it is desirable to avoid open chest surgery, and therefore impossible to establish firm contact with the wall of the atrium, other electrode configurations were suggested.
One lead configuration that has been suggested is the so-called "J-lead" which uses a flat metal contact in the form of a leaf spring which opens after the electrode is inserted in the heart.
Some of these configurations were either too dangerous (i.e., possible damage to the heart by forcing the contact through the thin wall of the atrium), or too difficult to insert, or interfered with the operation of the heart to an extent that they were never widely used. Other suggested systems, which did not establish reliable contact with the wall of the atrium, made it extremely difficult to detect a P wave in the presence of the ventricular signals and normal noise.
Noise, of course, also occurs in the ventricular electrode due to muscle artifact and the many sources of RF energy a patient is likely to encounter. For this reason, implanted pacers usually incorporate circuitry which, in the presence of excess ambient noise, causes the pacer to revert to a fixed rate mode.